1. Field of the Invention
The present invention relates to the etherification reaction of C4, C5 and/or C6 isoolefins with lower alcohols, such as methanol, to produce the corresponding tertiary ether. More particularly the invention relates to a process wherein a catalytic distillation process is carried out under conditions to avoid an azeotrope of alcohol with the hydrocarbons.
2. Related Information
Some ethers have been suggested to be detrimental as gasoline additives, because their presence has been detected in ground water. However, the ethers continue to be a valuable source of oxygenates to improve octane levels for reformulated gasoline. The increased governmental restriction on gasoline has strained the availability of feed stocks for etherification. The use of the isoamylenes for the preparation of octane improvers for gasoline has increased as has isohexene. It is highly desirable to be able to employ mixed isobutene/isoamylene and isobutene/isoamylene/isohexene streams.
The reaction of an alcohol and an olefin and concurrent separation of the reactants from the reaction products by fractional distillation has been practiced for some time. The process is variously described in U.S. Pat. Nos. 4,232,177; 4,307,254; 4,336,407; 4,504,687; 4,987,807; and 5,118,873. The isoolefins preferably react with the alcohol to form ethers. Briefly the alcohol and isoolefin are fed to a distillation column reactor having a distillation reaction zone containing a suitable catalyst, such as an acid cation exchange resin, in the form of a catalytic distillation structure, and also having a distillation zone containing an inert distillation structure. Tertiary olefins react preferably to the normal olefins.
U.S. Pat. No. 5,248,837 discloses a method for controlling catalytic distillation etherifications wherein the methanol concentration below the catalyst bed is controlled to a point that maximizes ether production and prevents alcohol from leaving with the bottoms ether product.
U.S. Pat. No.5,313,005 discloses a similar process to U.S. Pat. No. 5,248,837 wherein the alcohol content is controlled by total oxygen in the form of OH. This system is particularly useful when a mixture of alcohols is used.
In the etherification of olefins with alcohol heretofore it was preferable that an excess of the alcohol be available in the reactor. This means that there is an excess of methanol in the reaction distillation zone of the distillation column reactor. Under these conditions in the distillation column reactor the methanol forms a minimum boiling azeotrope with either of the olefins. In the case where C4 components are present the azeotrope is only slightly more volatile than the C4's alone, and therefore the methanol tends to remain in a relatively constant concentration with the C4's throughout the column. The concentration of the methanol in the C4 azeotrope is about 4% (depending upon the composition of the C4 mixture and operating pressure of the column), and it is necessary to operate with a methanol concentration to satisfy this azeotrope before the C5 azeotrope can be satisfied.
In maximized ether production reactions using excess alcohol the C5 azeotrope contains about 12 wt % methanol, and the boiling point of the azeotrope is 10 to 15° F. below that of the corresponding C5's, but above the C4's and C4/methanol azeotrope. If the net methanol flow into the column is higher than the azeotrope, the methanol concentration will increase (60% has been measured) until methanol leaves with the TAME bottoms product. Similar considerations apply to the C6 streams.
In addition to the considerations of azeotopes discussed above, when two or more different alcohols are fed at the same time, other factors affect the operation of the etherification. For example, in the case of etherification of iC5= with a mixed methanol/ethanol stream to produce TAME and tertiary amyl ethyl ether (TAEE), there are two different azeotropes. The first is methanol with the C5's which is 12% methanol. The second is the azeotrope between the C5's and the ethanol which is 8% ethanol. The different alcohols also react at different rates with the isoolefins, e.g., methanol reacts more rapidly than ethanol with isopentenes.
It is an advantage of the present invention that the alcohol/hydrocarbon azeotrope can be reduced or eliminated using the present process. It is a further advantage that the amount of ether in the gasoline mix can be reduced while enhancing the dimer production. The dimers (olefins) can be reduced or eliminated by hydrogenation, e.g., diisobutene hydrogenates to isooctane. Since the alcohol is maintained at the partial stoichiometric amount, it is another advantage that the process is easier to operate on a commercial scale, because the alcohol can be substantially eliminated as a down stream problem.